could i keep everything else and just be able to upgrade the cpu and gpu? like with the motherboard
yes but no sense at getting an unlocked "K" processor unless you will upgrade the motherboard later.
thre are i5-3330 but those are a bit slow, a i5-3470 or 3570 are a bit faster and is a very good gaming cpu. it might be better to look at a gtx760 getting a 7950 and trying to get a large overclock on it would mean upping the voltage and that could strain your 500 watt PSU.